For many years now displays have been designed for multiple users and optimised so that viewers can see the same good image quality from different angles with respect to the display. This assumes that the multiple users require the same information from the display. However, there are many applications where it would be desirable for the individual users to be able to see different information from the same display. For example in an automobile, the driver may wish to view satellite navigation data while the passenger may wish to view a movie. If two displays were used in this instance, it would be possible for the driver to view the movie, which might be distracting, and providing two displays would take up extra space and increase cost. In computer games, each player may wish to view the game from his or her own perspective. This is currently done by each player viewing their unique perspective on individual screens. This takes up a lot of space and is not practical for portable games.
By showing more than one image to more than one user on one display, there can be a considerable saving in space and cost. This may be desirable in airplanes where each passenger is provided with their own video screen. By providing one central screen for two or more passengers but retaining the ability to select their own movie, there could be a considerable saving in cost, space and weight. There is also the ability to preclude the users from seeing each other's views. This might be a desirable thing in security applications such as banking or sales transactions as well as games.
In normal vision, the two human eyes perceive views of the world from different perspectives due to their separate location within the head. These two perspectives are then used by the brain to assess the distance to the various objects in a scene. In order to build a display which will effectively display a three dimensional image, it is necessary to re-create this situation and supply a so-called “stereoscopic pair” of images, one to each eye of the observer.
Three dimensional displays are classified into two types depending on the method used to supply the different views to the eyes;                Stereoscopic displays typically display both of the images over a wide viewing area However, each of the views is encoded, for instance by colour, polarisation state or time of display, so that a filter system of glasses worn by the observer can separate the views and will only let each eye see the view that is intended for it.        Autostereoscopic displays require no viewing aids to be worn by the observer. Instead, the two views are only visible from defined regions of space. The region of space in which an image is visible across the whole of the display active area is termed a “viewing region”. If the observer is situated such that one of their eyes is in one viewing region and the other eye is in the viewing region for the other image of the pair, then a correct set of views will be seen and a three-dimensional image will be perceived.        
For flat panel autostereoscopic displays, the formation of the viewing regions is typically due to a combination of the pixel structure of the display unit and an optical element, generically termed a parallax optic. An example of such an optic is a parallax barrier, which is a screen with vertical transmissive slits separated by opaque regions. This screen can be set in front of a spatial light modulator (SLM) with a two-dimensional array of pixel apertures as shown in FIG. 1 of the accompanying drawings. The pitch of the slits in the parallax barrier is chosen to be close to an integer multiple of the pixel pitch of the SLM so that groups of columns of pixels are associated with a specific slit of the parallax barrier. FIG. 1 shows an SLM in which two pixel columns are associated with each slit of the parallax barrier.
The display shown in FIG. 1 comprises an SLM in the form of a liquid crystal device (LCD) having an active matrix thin film transistor (TFT) substrate 1 and a counter-substrate 2, between which are disposed a liquid crystal layer forming a picture element (pixel) plane 3 with associated electrodes and alignment layers (not shown) as appropriate. Viewing angle enhancement films 4 and polarisers 5 are provided on the outer surfaces of the substrates 1 and 2 and illumination 6 is supplied from a backlight (not shown). A parallax barrier comprises a substrate 7 with a barrier aperture array 8 formed on its surface adjacent the LCD and an anti-reflection (AR) coating 9 formed on the other surface thereof.
The pixels of the LCD are arranged as rows and columns with the pixel pitch in the row or horizontal direction being p. The aperture array 8 comprises vertical transmissive slits with a slit width of 2 w and a horizontal pitch b. The plane of the barrier aperture array 8 is spaced from the pixel plane 3 by a distance s.
In use, the display forms left and right viewing windows 10 and 11 in a window plane at the desired viewing distance of a display. Th e window plane is spaced from the plane of the aperture array 8 by a distance ro. The windows 10 and 11 are contiguous in the window plane and have a width and pitch e corresponding to the average human eye separation. The half angle to the centre of each window 10, 11 from the display normal is illustrated at α.
FIG. 2 of the accompanying drawings shows the angular zones of light created from an SLM 20 and parallax barrier 21 where the parallax barrier has a pitch of an exact integer multiple of the pixel column pitch. In this case, the angular zones coming from different locations across the display panel surface intermix and a pure zone of view for image 1 or image 2 does not exist. In order to address this, the pitch of the parallax optic is reduced slightly so that the angular zones converge at a pre-defined plane (termed the “window plane”) in front of the display. This change in the parallax optic pitch is termed “viewpoint correction” and the effect is illustrated in FIG. 3 of the accompanying drawings. The viewing regions, when created in this way, are roughly kite shaped in plan view.
For a colour display, each pixel is generally given a filter associated with one of the three primary colours. By controlling groups of three pixels each with a different colour filter, a large range of visible colours may be produced. In the autostereoscopic display, each of the stereoscopic image channels must contain sufficient of the colour filters for a balanced colour output. Many SLMs have the colour filters arranged in vertical columns, due to ease of manufacture, so that all the pixels in a given column have the same colour filter associated with them. If a parallax optic is disposed on such an SLM with three pixel columns associated with each slit or lenslet, then each viewing region will see pixels of one colour only. Care must be taken in the colour filter layout to avoid this situation and known examples of layouts are disclosed in EP 0752 609 and EP 0 770 889.
The function of the parallax optic is to restrict the light transmitted through the pixels to certain output angles. This restriction defines the angle of view of each of the pixel columns behind a given slit. The angular range of view of each pixel is decided by the refractive index of the glass, n, the pixel pitch, p, and the separation between the pixel and the parallax optic planes, s, in accordance with:
      sin    ⁢                  ⁢    α    =      n    ⁢                  ⁢    sin    ⁢                  ⁢          (              arctan        ⁡                  (                      p                          2              ⁢              s                                )                    )      
In order to increase the angle between viewing windows, it is necessary to increase the pixel pitch, p, decrease the gap between the parallax optic and the pixels, s, or increase the refractive index of the glass, n. Changing any of these variables is not easy. It is not always practical or cost-effective to significantly change the refractive index of the substrate glass. Pixel pitch is typically defined by the required resolution specification of the panel and therefore cannot be changed. Additionally, increasing pixel pitch requires a similar increase in the parallax barrier pitch which makes the barrier more visible, thus detracting from the final image quality. Decreasing s results in manufacturing problems associated with making and handling thin glass. Therefore, it is difficult to use a standard parallax barrier to create 3D or multi-view displays with wide viewing angles.
FIG. 4 of the accompanying drawings illustrates another known type of directional display in the form of a rear parallax barrier display. In the front parallax barrier display shown in FIG. 1, the parallax barrier is disposed between the SLM and the viewing windows 10 and 11 whereas, in the rear parallax barrier display shown in FIG. 4, the SLM is disposed between the parallax barrier and the viewing windows 10 and 11.
U.S. Pat. No. 6,424,323 discloses an image deflection system in the form of a lenticular screen which overlies a display device. The display is controlled so as to provide at least two independent images for viewing from different viewing positions. The images are interlaced in vertical rows.